Integrate code coverage (tarpaulin/llvm-cov)

[mroczect]

Summary

Set up code coverage reporting in the CI pipeline to track test coverage and ensure it stays above 80%.

Context

Coverage helps identify untested code paths and improve overall quality. cargo-tarpaulin or llvm-cov can be integrated with GitHub Actions.

Tasks

• Add a CI job using cargo-tarpaulin (or cargo llvm-cov) to generate coverage reports.
• Configure the job to:
  • Run on push and pull requests.
  • Execute all tests (cargo test --workspace).
  • Generate coverage (e.g., cargo tarpaulin --out Xml --output-dir coverage).
  • Upload coverage to Codecov or Coveralls (optional but recommended).
  • Enforce a minimum coverage threshold (e.g., 80%) using a coverage gate.
• Add a badge to the README showing the current coverage percentage.
• Document how to run coverage locally.

Acceptance Criteria

• Coverage job runs in CI and reports a percentage.
• The threshold is configured (fails if below 80%).
• Badge is added to README.

Dependencies

• All tests from previous phases.